The stabilization of polarons opens up new physics

Physicists at EPFL have developed a formulation to solve the long-standing problem of electron-self interaction when studying polarons – quasiparticles created by electron-phonon interactions in materials. The work may lead to unprecedented calculations of polarons in large systems, systematic studies of large groups of materials, and the molecular dynamics evolving over long periods of time.

One of the many peculiarities of quantum mechanics is that particles can also be described as waves. A common example is the photon, the particle associated with light.

In ordered structures, so-called crystals, electrons can be seen and described as waves that propagate over the entire system – a quite harmonious picture. As electrons move through the crystal, ions—atoms that carry a negative or positive charge—are periodically arranged in space.

Now if we were to add an extra electron to the crystal, its negative charge could cause the ions around it to move away from their equilibrium position. The electron charge would localize in space and couple to the surrounding structural “lattice” distortions of the crystal, creating a new particle known as a polaron.

“Technically, a polaron is a quasiparticle consisting of an electron ‘attracted’ by its self-induced phonons, which represent the quantized vibrations of the crystal,” says Stefano Falletta of EPFL’s School of Basic Sciences. “Polaron stability arises from a competition between two energy contributions: the gain due to charge localization and the cost due to lattice distortions. When the polaron is destabilized, the extra electron delocalizes throughout the system while the ions recover their equilibrium positions.”

In collaboration with Professor Alfredo Pasquarello from EPFL, they have published two articles in Physical Verification Letters and Physical Check B describes a new approach to solving a major flaw in an established theory that physicists use to study the interactions of electrons in materials. The method is called density functional theory or DFT and is used in physics, chemistry and materials science to study the electronic structure of many-particle systems such as atoms and molecules.

DFT is a powerful tool for performing ab initio calculations on materials by simplifying the treatment of electron interactions. However, the DFT is prone to unwanted interactions of the electron with its own self – what physicists call the “self-interaction problem”. This self-interaction is one of the major limitations of DFT and often leads to an incorrect description of polarons, which are often destabilized.

“In our work, we introduce a theoretical formulation for the electron-self interaction that solves the problem of polaron localization in density functional theory,” says Falletta. “This allows access to accurate polaron stabilities within a computationally efficient scheme. Our study paves the way to unprecedented calculations of polarons in large systems, in systematic studies with large sets of materials, or in molecular dynamics evolving over long time periods.”